Combustion engines are machines that convert chemical energy stored in fuel into mechanical energy useful for generating electricity, producing thrust, or otherwise doing work. These engines typically include several cooperative sections that contribute in some way to this energy conversion process. In gas turbine engines, air discharged from a compressor section and fuel introduced from a fuel supply are mixed together and burned in a combustion section. The products of combustion are harnessed and directed through a turbine section, where they expand and turn a central rotor.
A variety of combustor designs exist, with different designs being selected for suitability with a given engine and to achieve desired performance characteristics. One combustor design includes a centralized pilot nozzle and several main fuel injector nozzles, not shown, arranged circumferentially around the pilot nozzle. With that design, the nozzles are arranged to form a pilot flame zone and a mixing region. During operation, the pilot nozzle selectively produces a stable flame which is anchored in the pilot flame zone, while the main nozzles produce a mixed stream of fuel and air in the above-referenced mixing region. The stream of mixed fuel and air flows out of the mixing region, past the pilot flame zone, and into a main combustion zone, where additional combustion occurs. Energy released during combustion is captured by the downstream components to produce electricity or otherwise do work.
The primary air pollutants produced by gas turbines are oxides of nitrogen, carbon monoxide and unburned hydrocarbons. For many years now, the typical combustor has included a primary injection system at a front end thereof to introduce fuel into the combustion chamber along with compressed air from compressor section. Typically, the fuel and air are premixed and then introduced into an igniter to produce a flowing combustion stream that travels along a length of the combustion chamber and through the transition piece to the first row of turbine blades. One challenge in such single site injection systems is there is always a balance to be obtained between the combustion temperature and the efficiency of the combustor. The amount of energy released during combustion is a product of many factors, including the temperature at which the combustion takes place, with increases in combustion temperature generally resulting in increased energy release. However, while increasing the combustion temperature can produce increased energy levels, it can also have negative results, including increased production of unwanted emissions, such as oxides of nitrogen (NOx), for which overall levels are directly related to the length of time spent at elevated temperatures. While high temperatures generally provide greater combustion efficiency, the high temperatures also produce higher levels of NOx.
Recently, combustors have been developed that also introduce a secondary fuel into the combustor. For example, U.S. Pat. Nos. 6,047,550, 6,192,688, 6,418,725, and 6,868,676, all disclose secondary fuel injection systems for introducing a secondary air/fuel mixture downstream from a primary injection source into the compressed air stream traveling down a length of the combustor. These systems introduce fuel at a later point in the combustion process and reduce at least some NOx levels by shortening the residence time of the added fuel with respect to the primary fuel and by maintaining an overall-lower combustion temperature by adding less fuel at the head end. However, even with these advancements, there remains a need for a secondary fuel supply system specifically designed to address the excessive levels of vibration found in some sections of the engine, like the transition piece. The transition piece can, for example, be a difficult place in which to mount a secondary fuel delivery system, because it is prone to especially-high levels of vibration, and placing known secondary fuel delivery systems there will subject them to forces which, if not addressed, can lead to excessive wear and can cause premature failure. Use of traditional vibration reduction methods, such as increasing component mass to improve stiffness, present additional difficulties when applied to the transition section, because the additional bulk is not only difficult to cool, but it can also interfere with the delicate aerodynamic characteristics of the flow path, leading to overall losses in efficiency and/or performance issues. Therefore, there still remains a need in this field for a fuel delivery system that, in addition to providing a supply of fuel and/or diluent to a secondary combustion region in the transition piece, downstream of a primary combustion zone, also includes features that address elevated levels of vibration, while maintaining sufficient cooling in the area surrounding the secondary combustion zone.